In recent years, cancer immunotherapy using cytokines has received attention. For example, therapeutic strategies using gene introduction have been explored for treatment of glioblastoma (Glioblastoma multiforme; GBM) (Shapiro, W. R., Arch. Neurol., 56: 429-432, 1999), which is one of the malignant brain tumors that has been thought to be untreatable in spite of various approaches including surgery, radiotherapy, and chemotherapy (Ram, Z. et al., Cancer Res., 53: 83-88, 1993; Sampson, J. H. et al., Proc. Natl. Acad. Sci. USA, 93: 10399-10404, 1996; Herrlinger, U. et al., Cancer Gene Ther., 4: 345-352, 1997; Seleh, M. et al., J. Natl. Cancer Inst., 91: 438-445, 1999; Giezeman-Smits, K. M. et al., Cancer Res., 60: 2449-2457, 2000). Some gene therapy strategies are expected to be effective based on in vivo animal model studies. However, low gene introduction efficiency limits their therapeutic effects in almost all cases. Major obstacles to a successful use of such gene therapy strategies are inability of recombinant viral vectors to spread to the whole tumor mass and low efficiency of in vivo introduction (Ram, Z. et al., Nat. Med., 3: 1354-1361, 1997). To promote gene therapy, development of a new vector system that is capable of safely and efficiently introducing genes into target cells is needed.    Non-patent Document 1: Shapiro, W. R., Arch. Neurol., 56: 429-432, 1999    Non-patent Document 2: Ram, Z. et al., Cancer Res., 53: 83-88, 1993    Non-patent Document 3: Sampson, J. H. et al., Proc. Natl. Acad. Sci. USA, 93: 10399-10404, 1996    Non-patent Document 4: Herrlinger, U. et al., Cancer Gene Ther., 4: 345-352, 1997    Non-patent Document 5: Seleh, M. et al., J. Natl. Cancer Inst., 91: 438-445, 1999    Non-patent Document 6: Giezeman-Smits, K. M. et al., Cancer Res., 60: 2449-2457, 2000    Non-patent Document 7: Ram, Z. et al., Nat. Med., 3: 1354-1361, 1997